The analysis of samples, especially powder samples, by X-ray diffraction is a very useful technique for determining the composition of the sample.
X-ray diffraction typically uses radiation emitted from an X-ray source which uses a target of a single predetermined element. Targets include cobalt, chromium, copper, molybdenum and silver.
The X-rays emitted by such targets are not fully monochromatic, as each target can emit radiation in a number of lines in addition to a continuous Bremsstrahlung spectrum. In particular, targets can emit radiation from a K line which may be subdivided into a K-alpha and a K-beta line.
Typically, more energy is emitted in the K-alpha line than the K-beta line so X-rays are often passed through a monochromator or beta-radiation filter to select the K-alpha line which is then used for X-ray diffraction measurements.
At sufficient energy resolutions, the K-alpha line is in fact a doublet, and resolves into two lines, a K-alpha-1 and a K-alpha-2 line with an intensity ratio of around 2:1. Accordingly, for high resolution X-ray diffraction measurements it is necessary to use a monochromator which is sufficient to select one of these lines to ensure that the X-rays used are sufficiently monochromatic. Bragg reflections may be used.
In view of the very high alignment accuracy required for X-ray diffraction measurements, the number of components that need to be aligned causes considerable difficulty in setting up and maintaining X-ray diffraction apparatus. Moreover, the intensity of X-rays may be reduced in such arrangements in order to achieve sufficient monochromaticity, since in conventional arrangements high resolution monochromators select only a very narrow solid angle range of X-rays emitted from the X-ray source.
An apparatus which addresses such considerations is disclosed in US2011/0268252, which describes apparatus suitable for X-ray diffraction measurements of powder samples with a complex arrangement for creating a virtual source with a very narrow energy bandwidth, i.e. with a monochromator that is sufficiently precise that it can separate out the K-alpha-1 and K-alpha-2 lines so that measurement is only made using one of these lines.
There is a desire for methods and apparatus for X-ray diffraction which are capable of achieving similar accuracy and resolution without the need for such accurate monochromators capable of selecting the K-alpha-1 from the K-alpha-2 line.